Roof bolts are well known in the mining industry and are extensively employed for consolidating the roof and providing anchoring points and support.
Their use involves drilling a hole in the roof of the mine and inserting into the hole a resin filled cartridge. The resin filled cartridges are also well known and comprise a curable resin and a catalyst for the resin contained in a sausage-like skin. In the next step of the installation a bolt is inserted which pierces the skin and penetrates the resin. The bolt is then usually spun about its axis to mix the resin and catalyst and, once mixed, the resin cures and sets.
In mines in some parts of the world for example in Australia it is the normal practice to apply tension to the bolt after the resin has set. This may is done by providing a screw threaded portion on the bolt and screwing a nut onto the threaded portion to abut a bearing plate against the rock surface. The nut is tightened until a chosen torque is reached, which is normally 120 to 150 ft lbs. This results in tension between the anchored part of the bolt and the bearing plate.
The final step in the installation procedure is therefore to tighten up the bolt against the plate. However the nut is usually locked onto the bolt so that the nut and bolt can be rotated together to mix the resin during the mixing stage of the installation and a mechanism is provided to cause the nut to break out, as it is called, when a certain torque leas been reached. In this way, once the bolt is securely anchored by the resin, further rotation of the nut, usually by means of a drilling machine, causes the locking mechanism to break and the nut can then be tightened up against the bearing plate. Many different nut break out systems are already known in the art. An example is described in U.S. Pat. No. 6,296,429 B1.
In countries such as the USA, the usual practice is to employ a bolt that has neither a thread nor a nut but which has a head by means of which it is rotated in the hole. Applying tension by tightening up a nut on the bolt is not usually considered necessary.
Problem To Be Solved By the Invention
For both bolts which are tensioned by tightening up a nut and those which are not, there is a long standing problem which is called glove fingering.
Insertion of the bolt into the hole containing the cartridge causes pressure on the cartridge which forces the skin to the hole wall. The bolt bores a hole through the contents of the cartridge leaving the skin substantially intact This results in a reduced direct contact by the resin with the wall of the hole and hence a less than optimum anchorage.
As mentioned above, in the USA bolts are usually employed having neither a thread nor a nut and in mining operations in the USA a frequently used bolt is one which is about ⅝ (five eighths) of an inch in diameter routinely employed in a hole which is about 1 (one) inch in diameter. Another frequently used combination in the USA is a ⅞ (seven eighths) inch diameter bolt in a 1 (one) and ⅜ (three eighths) inch diameter hole.
In Australia using a threaded bolt which is subsequently tensioned, a typical operation will use a bolt which is 22 mm in diameter in a hole of 28 mm in diameter. Glove fingering is a problem encountered with all of these combinations of sizes of bolt and hole.
Previous attempts to solve the problem have involved the use of a thinner skin to contain the resin, coarse filler in the cartridge and slash cut bolts. However these have generally achieved only limited success.
Our pending U.S. Provisional Patent Application No. 60/318637 describes a solution to the problem for bolts having a head and which are not subsequently tensioned by causing the bolt, when it is rotated or spun by means of its head, to describe a circle of diameter greater then its own diameter. One embodiment of the invention described in that application is a novel form of roof bolt in which the head of the bolt is offset with respect to the axis of its shaft by at least 0.08 inches.
The present invention provides a solution to the problem of glove fingering in the case of threaded bolts which are to be tensioned by means of a nut. The problem is solved by a novel construction of nut which causes the bolt, when rotated, to describe a circle of diameter greater than its own diameter.